1. Field of the Invention
The present invention relates to a method of removing a resist mask and a method of manufacturing a semiconductor device, and more particularly to a method of removing a photoresist mask or a photosensitive polyimide mask which remains after implanting impurity ions selectively into a semiconductor substrate to form a conductive layer or a steel or the like to improve hardness and persistence and a method of manufacturing a semiconductor device including a process of removing a resist mask.
In a method of manufacturing a semiconductor device, it is required to introduce impurities such as boron and phosphorus into a semiconductor substrate or the like selectively by ion implantation in order to form a pn junction such as a source/drain region and the other conductive layers in a semiconductor layer, a semiconductor substrate or the like.
In such a case, such a process is to form a mask using a photoresist or a photosensitive polyimide and to remove the mask after ion implantation is performed.
2. Description of the Prior Art
FIG. 1A to FIG. 1E are sectional views showing how to remove a photoresist mask after conducting ion implantation selectively into a semiconductor substrate for instance using a photoresist mask.
First, a photoresist mask is formed on a semiconductor substrate 1 by spin coating. Then, after exposing the photoresist film, it is soaked in a developer and a photoresist mask 2 is formed as shown in FIG. 1A.
Next, as shown in FIG. 1B, impurity ions are implanted selectively into the semiconductor substrate 1 through the photoresist mask 2.
At the same time, impurities, such as P, B and As, are bonded chemically with the remaining photoresist polymer, and the surface layer of the photoresist mask 2 changes in quality and becomes a very hard layer. This hard layer is referred to as a carbonized layer (transmuted layer) 2b. Besides, the photoresist polymer inside the photoresist mask beyond reach of impurities remains in a state as it is. This interior layer is referred to as an un-changed layer (non-transmuted layer) 2a.
Then, ashing, using an oxygen plasma, is applied to the photoresist mask 2 so as to remove it.
Now, the ashing, using oxygen plasma, demolishes the carbonized layer 2b in the surface layer of the photoresist mask 2 by utilizing bombardment by ions in oxygen plasma, and also the photoresist mask is removed by utilizing an oxidation reaction between oxygen ions and the photoresist polymer.
As shown in FIG. 1C, however, in ashing with oxygen plasma, impurities (such as P, B and As) implanted into the photoresist mask 2 and oxygen formed into plasma react with each other, and oxides of these impurities are produced. In particular, there is such a problem that those oxides of impurities formed in the ashing of the photoresist mask after ions are implanted at high dose, are difficult to volatilize. Therefore they remain on the substrate as it is.
Further, when the wafer temperature rises, because of plasma irradiation, before the carbonized layer 2b is removed completely, volatile components inside the un-changed layer 2a are gasified and expanded as shown in FIG. 1D. In such a case, since the carbonized layer 2b which covers the un-changed layer 2a has a structure which is dense and does not pass gas 2c, what is called a "photoresist explosion", in which the carbonized layer 2b explodes becasue of the pressure of the expanded gas 2c, is generated as shown in FIG. 1E. Further, there is such a problem that the pieces of the carbonized layer 2b that have scattered by the photoresist explosion form particles, which cause lowering of production yield of semiconductor devices.
Accordingly, a method of performing hydrogen plasma processing, such that the wafer temperature is maintained at a low temperature, has been developed as a process in which the photoresist explosion is not brought about, and oxides of impurities are not produced. This processing method is called a 2-step ashing process, and has been announced already by the present inventor under a title of "High Ashing Rate of Ion Implanted Resist Layer" in DRY PROCESS SYMPOSIUM OF 1992.
It has been well known that a compound of an impurity (P, B or As) and hydrogen is liable to volatilize, and it was found that the carbonized layer had been removed without leaving residues when hydrogen plasma processing was performed practically. Further, since the wafer is cooled to a low temperature of approximately 5.degree. C., the photoresist explosion has not occurred. Reactive ion etching (RIE) for removing the carbonized layer physically and chemically by hydrogen ions is suitable for the hydrogen plasma processing.
Further, after the carbonized layer is removed by the hydrogen plasma processing, the interior un-changed layer appears. In order to remove this un-changed layer, a downstream ashing process using O.sub.2 as main reaction gas is very often applied because its process has less bombardment of the substrate by ions.
By applying the 2-step ashing processing in which the plasma processing by hydrogen gas and the downstream processing by oxygen gas described above are performed in succession, generation of oxides of impurities is prevented, and photoresist explosion is also prevented, thus greatly reducing the quantity of produced particles. With this, it is possible to aim at improvement of production yield of semiconductor devices.
When an ashing process is performed using this 2-step ashing process, however, new residues 4 remain along the side wall of the removed photoresist mask 2 sometimes as shown in FIG. 2, which becomes a problem.